Enhance ping probe testing

[manugarg]

Improve ping probe tests to cover cases like the one reported in: #225.

[manugarg]

Just want to add that detecting #225 through unit tests was really hard because that bug is triggered only if we receive packets smaller than the configured packet size (payload size + icmp header size). This happens with raw sockets in the wild because kernel delivers all kind of ICMP packets to us and those packets may be smaller than what cloudprober would send.

Given this specific nature of #225, I'd consider this bug to be lower priority.

[manugarg]

Closing this now, as noted earlier this may not be worth fixing.